Thermal Decomposition of 3-Nitro-1,2,4-Triazole-5-One (NTO) and Nanosize NTO Catalyzed by NiFe2O4

Nanosize Nickel ferrite (NiF) was synthesized by the co-precipitation methods, and its effect as a 5% by mass additive was studied on the thermal decomposition of micrometer and nanometer size NTO. In the presence of a 5% NiF additive, the thermal decomposition peak temperature of NTO was decreased from 276.36 to 260.18 °C and that of nanoNTO was decreased from 261.38 to 258.89 °C (β = 10 °C min−1). The kinetics parameters confirm the catalytic activity of NiF for the thermal decomposition of NTO, and nNTO as the parameters such as activation energy (NTO =  ~ 25.45% and nNTO =  ~ 45.94% decrement), and pre-exponential factor (NTO =  ~ 21.94% and nNTO =  ~ 43.12% decrement) were decreased when 5% NiF additive was added to NTO, and nNTO. The rate of the decomposition process was increased in the presence of a 5% NiF catalyst, indicating the faster thermal decomposition of both NTO, and nNTO in the presence of a nickel ferrite catalyst.


Introduction
High energetic materials (HEMs) are a special class of chemical components that are used for the applications such as construction, rocket propellants, missiles, gunpowder and mining as HEMs release immense energy and gases that can be utilized to thrust an object or destroy a target. The examples of such HEMs include ammonium perchlorate, ammonium nitrate, 1,3,5,7-tetranitro1,3,5,7-tetraazacyclooctane (HMX), 1,3,5-trinitro-1,3,5-triazinane (RDX), 3-nitro-1,2,4triazole-5-one (NTO) [1]. NTO (Impact sensitivity 57.5 H 50 cm −1 ) is one of the most potential candidates for replacing the otherwise sensitive HEMs such as HMX (Impact sensitivity 22.4 H 50 cm −1 ), RDX (Impact sensitivity 25.6 H 50 cm −1 ) as it is highly insensitive to external stimuli [2]. NTO is the main ingredient of the well-known insensitive munitions formulation IMX-101, which has previously been certified by the US Army as a safer and similarly powerful substitute for 2,4,6-trinitrotoluene. NTO can decrease the impact and sensitivity of other HEMs like HMX [3]. During military applications, weapons are vulnerable to unforeseen external stimuli, which can pose significant risks in the transport, storage, and application of explosives. Thermal stimulation is one of the most prevalent external stimuli that can cause explosives to decompose and detonate [4]. However, research has been carried out to improve the thermal decomposition performance of NTO which can directly influence the burning and combustion performance of explosive compositions. The thermal properties of NTO can be improved by the use of (i) additives [5,6], (ii) nanosizing of NTO [7,8], (iii) and/or Forming the co-crystals with other HEMs such as HMX. The various literature [9][10][11] reported that metal oxide (MO)-based nanomaterials can significantly improve the thermal decomposition of HEMs such as AP, HMX, RDX due to the smaller size and large surface-to-volume ratio of MOs. i.e., Wei et al. [12] reported that Bi 2 WO 4 nanomaterial with the particle size of 40 nm shows better results on the thermal decomposition of HEMs such as RDX, HMX, ammonium perchlorate, and double base propellants as well as the burning rate of double-base propellant compare to 100 nm size Bi 2 WO 4 . MOs can facilitate the electron/proton transfer during the decomposition of HEMs as metals such as transition metals can exist in a variety of oxidation states and absorb the gaseous generated during the decomposition catalyzing the intermediate reac-tions. However, only a few studies have reported the effect of transition metal-based additives on the thermal decomposition of NTO. A study by Prabhakaran et al. [6] shows the effect of various TMOs as a 5% by mass additive and found that TMOs can be used to improve the exothermic peak temperature of NTO, however, the kinetic parameters were not studied which play an important role in determining the catalytic activity. Hanafi et al. [5] studied that incorporating the 2D energetic hybrid crystals containing metals such as Zn, and Co not only decreases the peak temperature of an exothermic curve of NTO, but also decreases high the activation energy of NTO with enhanced heat of decomposition. A density functional theory was studied by Gorb et al. [13] to improve the decomposition of NTO in the presence of Fe 13 O 13 nanoparticles. The authors have predicted a possible interaction between NTO may be accompanied by a barrierless electron movement from nanoparticle to NTO [13]. The previous studies have reported that AB 2 O 4 type metal oxides such as MFe 2 O 4 (where A M Co 2+ , Cu 2+ , Ni 2+ , etc. B Fe, Co etc.) impart good catalytic activity for decreasing the thermal decomposition temperature and activation energy of the decomposition of HEMs such as ammonium perchlorate, ammonium nitrate along with enhancing the burning rate of the corresponding solid rocket propellants [14,15]. The ease of synthesis, utilization of less toxic and cost-effective chemicals, less energy consumption during the synthesis, and magnetic separation of ferrites give them an advantage over other costly catalysts/additives. Previously an article [16] suggested that Metal salts of NTO such as Fe-NTO, and Ni-NTO can efficiently improve the burning rate of Composite propellants.
In the present study, we have reported the application of 5% by mass nanosize nickel ferrite (NiFe 2 O 4 or NiF) catalyst for improving the thermal decomposition and kinetic parameters such as activation energy, the pre-exponential factor of NTO, and nanosize NTO (nNTO). The TG-DSC data were used for the evaluation of the effect of catalyst on the decomposition of NTO, and nNTO. Three isoconversion methods, namely Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), and Starink method were used for calculating the kinetic parameters.

Materials and methodology
Metal nitrate salts and sodium hydroxide used in the present work were acquired from local providers and used as such without further purification. NTO was synthesized in the laboratory using the previously reported method [17]. Nanosize NTO was prepared using the solvent (tetrahydrofuran)antisolvent (n-hexane) approach [18].

Synthesis
Wet chemical precipitation method was used to synthesize nanosize NiF. Briefly, Ni(NO 3 ) 2 ·6H 2 O (8.72 g) and Fe(NO 3 ) 3 ·9H 2 O (24.24 g) in 1:2 mol ratio were mixed in 100 mL distilled water. To this mixture 2 M, 125 mL NaOH was added under magnetic stirring to obtain the black precipitates of corresponding hydroxide salts [Ni(OH) 2 and Fe(OH) 3 ]. After completion of the precipitation (pH~11-12), the precipitation was filtered, washed with hot water to wash away impurities, and dried in a hot air oven at 60°C. Finally, the calcination of the precipitates at 500°C for 5 h results in a muffle furnace that yields nanosize NiF.
0.095 g NTO and 0.005 g NiF were mixed mechanically using a mortar-pestle to yield NTO + NiF composition. Similarly, 0.095 g nNTO and 0.005 g NiF were mixed mechanically using a mortar-pestle to yield nNTO + NiF composition. The four compositions (NTO, nNTO, NTO + NiF, and nNTO + NiF) were used for the decomposition study using TG-DSC data.

Characterization
The structure of NiF was confirmed using powder X-ray diffraction (XRD, Ultima IV Powder X-ray Diffractometer instrument equipped with a Cu Kα radiation source; λ 1.5406 Å) and Raman analysis (JobinYvon Horiba LabRam, HR800 laser source; λ 532 nm).

Catalytic Activity
Thermal decomposition of NTO compositions was evaluated using TG-DSC (Perkin Elmer, STA 8000) at three linear heating rates (5, 10, and 15°C min −1 or K min −1 ) from room temperature to 400°C temperature under an inert nitrogen atmosphere in a platinum pan. The obtained data were used to calculate the decomposition kinetic parameters of four NTO compositions.

Results and Discussion
The XRD patterns of NiF were compared with the COD database file (5910064; Reference code: 96-591-0065) to confirm the formation of ferrite. The XRD spectrum of NiF is provided in Fig. 1a (440) planes. NiF has a cubic structure with F d-3 m (227) space group. The XRD data were used to calculate the lattice parameter (a, Eq. 1), cell volume (a 3 ; V), and X-ray density (ρ X-ray , Eq. 2). William-Hall equation 3) was used to evaluate the lattice strain and crystalline size of NiF. Interception and slope of the plot of 4sinθ on X-axis against βcosθ on Y-axis give the value of crystalline size and lattice strain, respectively (Fig. 1b).
where a is the lattice parameter (Å), d is the inter-atomic spacing (Å), and h, k, and l are the miller indices. M represents the molar mass of NiF (234.38 g mol −1 ), N A is the Avogadro's number (6.022 × 10 23 mol −1 ), k is the constant value of which is usually taken as 0.9, λ is the wavelength of the radiation source (0.15406 nm for Cu K−α), D is the crystalline size (nm), ε is the lattice strain, β is the full width at half maxima of the given XRD peak, and θ is the Bragg's angle (2θ /2 in degree). The parameters calculated from the powder XRD data for NiF are depicted in Table 1. The lattice parameters were close to the previously reported values, and so was the X-ray density. The general trend was found from the comparison that as the X-ray density was increased, the size of NiF was also increased as the larger size particles require less volume for the same mass of the substance. However, not all results were according to this trend [19]. The Raman spectrum (Fig. S1) (3), and A 1g active modes, respectively. The Raman peaks of NTO, and nNTO with and without 5% by mass NiF additive exhibit a similar pattern. The broadening of the Raman peaks was observed when NiF was added to NTO, and nNTO. The broadening could be because of the nanosize of NiF as peaks are broadened in nanosize material compared to micron size material [22].

Thermal Decomposition
The TG curve of four NTO compositions is given in Fig. 2. The decomposition of NTO takes place in a single step at~259°C and that of nNTO takes place at~256°C. When 5% by mass NiF is mixed, the Decomposition of both NTO, and nNTO occurs in two steps. The first mass loss occurs~200°C with a mass loss of 5.17% in the case of NTO + NiF and 6.98% in the case of nNTO + NiF. The second mass loss for both NTO + NiF, and nNTO + NiF occurs between~200-260°C with mass loss of 63.75% and 64.08%, respectively. The thermal stability order was: NTO > nNTO > NTO + NiF > nNTO + NiF. The stability of both NTO and nNTO was decreased in the presence of NiF catalyst. Although the initial decomposition step of NTO is not clear, it is believed to proceed through H-abstraction (from N-H) and cleavage of C-NO 2 bond. Hence, it is proposed that NiF can abstract H from N-H bond and facilitate the cleavage of C-NO 2 bond. NiF can adsorb the gases released during the decomposition of NTO catalyzing the decomposition reaction. The DSC curve (Fig. 3) of NTO shows a single exothermic peak corresponding to the decomposition of NTO at~250-280°C with a 276.36°C peak temperature. In the presence of NiF catalyst, the DSC curve of NTO shifts to a lower temperature (i.e.,~240-270°C) with a peak temperature of 260.18°C. The decrement in the peak temperature of the high-temperature exothermic curve was 16.18°C, the decrement is good as unlike ammonium perchlorate, NTO is not highly affected by the presence of additives [6]. Similarly, the exothermic DSC curve of nNTO was decreased by 2.49°C. The effect of NiF on the DSC curve of nNTO was not as great as NTO, this could be because nNTO's decomposition takes place at a lower temperature compared to micron size NTO because of the reduced size. Both NTO + NiF, and nNTO + NiF exhibit similar DSC curves with a slight difference in the decomposition temperature. (i.e., decomposition of nNTO + NiF is~2°lower than NTO + NiF for both exothermic curves). The additional DSC exothermic curve at~190-210°C was observed in the presence of NiF additive.

Kinetics Parameters
The kinetic parameters were investigated using changing the DSC curve data at 5, 10, and 15°C min −1 . The effect of heating rates on the thermal decomposition curve of all four samples is depicted in Fig. 4. The DSC curves of all compositions were shifted to higher temperature values with increasing heating rates. i.e., the decomposition of NTO was completed at a higher temperature value at a higher heating rate. The DSC curves for all samples corresponding to the exothermic curve (~240-280°C) were observed at all heating rates, but the low-temperature exothermic curve (NTO + NiF, and NTO + NiF) was found to be almost diminishing at a higher heating rate (15°C min −1 ).
The activation energy of NTO, and nNTO with and without NiF nanocatalyst is given in Table 2 and the variation in the log A and E from conversion value 0.1-0.9 at an interval of 0.025 are depicted in Fig. 5. The activation energy and pre-exponential factor calculated using FWO method were higher compared to KAS, and Starink methods. The activation energy of NTO was high for initial decomposition; however, the activation energy was decreased drastically with the progress of the decomposition. In the case of nNTO, the activation energy does decrease as the decomposition proceeds, but the change was not as drastic as NTO. When NiF catalyst was present, the energy for the initial decomposition was lower, but as the decomposition proceed, the activation energy was increased. i.e., the activation energy of NTO + NiF, and nNTO + NiF after α 0.5 was higher than NTO. The average activation energy of NTO was decreased by~99 kJ mol −1 , and that of nNTO was decreased by 224.75 kJ mol −1 when NiF catalyst was present. In the presence of NiF, the pre-exponential factor of NTO, and nNTO was decreased largely indicating faster decomposition. The     [5] applied four energetic highly energetic coordination polymers of triaminoguanidine-transition metal (Co or Zn) (T-Co, or T-Zn) complexes, with or without graphene oxide (GO)-T for improving the thermal decomposition performance of NTO. 20% by mass energetic catalysts were able to reduce the DSC peak temperature of NTO up to 13.3°C, similarly, in another study [6], the lead monoxide was able to decrease the peak temperature of NTO by 8°C. In the present work, NiF was able to reduce the decomposition of NTO by 16.18°C.
The major products of NTO breakdown are CO 2, H 2 N 2 , N 2 , NH 3 , H 3 N 3, HN 2 . and H 2 O. Molecular dynamics simulations suggest NTO breakdown takes place by direct ring fracture. Direct rupture of the five-membered ring causes a molecular collapse in the unimolecular breakdown of NTO [27,28]. The ring rupture, C-NO 2 elimination, and Habstraction are the potential pathways through which NTO decomposes. Still, the decomposition mechanism of NTO is debatable with no straight answers. The NiF nanoparticles possess hydroxyl (-OH) on their surface, and hence, possess a negative surface which may help in the H transfer as well as unfilled d-orbitals of nickel and iron may help in the electron transfer to facilitate the decomposition of NTO.

Conclusions
The synthesized NiF nanoparticles imparted great catalytic activity for decreasing the thermal decomposition temperature of both nanometers as well as micrometer NTO. The addition of only 5% by mass NiF nanocatalyst was able to decrease the DSC peak temperature of nNTO by 2.49°C, and that of NTO by 16.18°C. The decreased decomposition temperature can make the decomposition NTO faster, and therefore can improve its decomposition performance. The DSC curves of NTO, and nNTO show only one exothermic peak corresponding to the thermal decomposition (> 260°C), however, in the presence of the nanocatalyst, two peaks were observed one peak~198°C, and a second peak at > 250°C. The decreased temperature of TG curves in the presence of nanocatalyst indicates that the stability of the compositions of NTO, and nNTO containing 5% catalyst was also decreased.